This application, and the innovations and related subject matter disclosed herein, (collectively referred to as the “disclosure”) generally concern audio systems to remove echo from a near-end signal, and more particularly but not exclusively, to filter (as opposed to merely suppressing) non-linear components of echo. Some, but not all, disclosed principles can be embodied as an audio device having a loudspeaker transducer to render a far-end signal, an external microphone to receive a near-end audio content (e.g., speech), and an internal microphone positioned to receive an indication of the loudspeaker transducer's output, which may non-linearly correspond to the far-end signal. The audio device can also have an echo canceller to filter a signal (e.g., after appropriate delays) generated by the internal microphone from a signal generated by the external microphone to provide a cleaner representation of the near-end content relative to the unfiltered signal generated by the external microphone.
Conventional audio devices have a loudspeaker transducer and a microphone transducer. The loudspeaker transducer can emit sound corresponding to a far-end acoustic signal and the microphone transducer can emit an acoustic signal corresponding to near-end audio (acoustic waves) received by the microphone transducer.
The sound emitted by the loudspeaker can reverberate within a room or other environment surrounding the audio device. After a finite delay, the microphone can receive reverberations deriving from the sound emitted by the loudspeaker, which can impair an intended near-end audio. The reverberations are sometimes referred to in the art as “echo.” An echo canceller can filter an estimate of the reverberation from the near-end audio signal observed by the microphone to provide a less-impaired near-end signal representative of the intended near-end audio.
Before reaching the loudspeaker transducer, the far-end acoustic signal can pass through a digital-to-analog converter (DAC) and the resulting analog signal can be amplified. Conventional acoustic echo cancellers (AEC) use the far-end acoustic signal taken just prior to the DAC, with appropriate gains and delays, as an estimate (reference) of the far-end signal to be filtered from the observed (e.g., impaired) near-end signal emitted by the microphone.
However, the sound emitted by the loudspeaker transducer usually has components that do not linearly correlate to the reference signal. For example, the DAC and amplification functions can introduce non-linear effects into the signal rendered by the loudspeaker transducer, e.g., an over-driven amplifier can introduce non-linear distortions. Further, a loudspeaker driver's force factor (i.e., a measure of a driver motor's electro-motive force applied to a loudspeaker diaphragm per unit of electrical current) can vary non-linearly over large diaphragm displacements, or excursions, as occurs when a loudspeaker transducer is driven under high amplification. As well, a restorative force applied to the diaphragm by a loudspeaker's suspension can vary non-linearly with displacement over large excursions. Consequently, a loudspeaker can introduce non-linear distortions under high amplification rates. And, mechanical vibrations induced in an enclosure for the audio device by output from the loudspeaker transducer can further impair the near-end signal observed by the microphone transducer. Other non-linearities also can arise.
Thus, even after a conventional AEC filters the estimated far-end signal from the observed near-end signal, non-linear components remain and require further signal processing to suppress the corresponding non-linear effects. However, such residual echo suppression can introduce further distortion into the near-end signal.
Accordingly, a need remains for an audio system capable of filtering linear and non-linear components of echo from a near-end signal.